Short answer
"The heart relaxes to fill with blood." is the normal sentence. We do not need "the" because we mean blood in general. "The heart relaxes to fill with the blood." sounds odd unless you already talked about a specific, known blood.
Step-by-step explanation
- General vs. specific: The word "the" is called a definite article. We use "the" when we mean a particular thing that both the speaker and listener know about. Without "the" we usually talk about something in general.
- Blood is usually general here: When you say "The heart relaxes to fill with blood," you are explaining how all hearts normally fill with blood. That is general information, so we don’t use "the."
- When to use "the blood": You would use "the blood" if you already mentioned a specific batch of blood. Example: "We put a sample in the tube. The heart fills with the blood from the sample." This sounds strange for a real heart, but it shows the idea: "the blood" points to a particular blood you have in mind.
Everyday examples
- General: "The sponge filled with water." (any water)
- Specific: "The sponge filled with the water from the sink." (that particular water)
One quick tip
For school or science facts about bodies, use: The heart relaxes to fill with blood. Also, write "heart" with normal lower-case letters, not all caps.
Better sentence if you want to be clearer
You can also say: "When the heart relaxes, it fills with blood." This sounds natural and clear.